The ligand binding site of the platelet integrin receptor GPIIb-IIIa is proximal to the second calcium binding domain of its alpha subunit.
The extreme carboxyl-terminal amino acid sequence of the gamma chain of fibrinogen is involved in the binding of this adhesive protein to the platelet integrin glycoprotein (GP) IIb-IIIa, and synthetic peptides corresponding to this region inhibit fibrinogen as well as fibronectin and von Willebrand factor binding to platelets. A chemical cross-linking approach was used to characterize the interaction of a 16-amino acid fibrinogen gamma chain peptide with platelets and to localize the site of its binding to GPIIb-IIIa. This peptide became specifically cross-linked to GPIIb, and platelet stimulation selectively enhanced its cross-linking to this alpha subunit. The cross-linking reaction was specifically inhibited by fibrinogen and an Arg-Gly-Asp peptide but not by an unrelated protein or a substituted peptide. Utilizing a combination of immunochemical mapping, enzymatic and chemical digestions, and amino acid sequencing, the cross-linking site of the gamma chain peptide in GPIIb was localized to a stretch of 21 amino acids. The identified region, GPIIb 294-314, contains the second putative calcium binding domain within GPIIb. The primary structure of this region is highly conserved among alpha subunits of other integrin adhesion receptors. These results identify a discrete region of GPIIb that resides in close proximity to a ligand binding site within GPIIb-IIIa. The homologous region may be involved in the functions of other integrin receptors.
D'Souza
S E
SE
Committee on Vascular Biology, Research Institute of Scripps Clinic, La Jolla, California 92037.
Ginsberg
M H
MH
Burke
T A
TA
Plow
E F
EF
D000595:Amino Acid Sequence; D001665:Binding Sites; D002118:Calcium / Q000097:blood*; D004591:Electrophoresis, Polyacrylamide Gel; D005340:Fibrinogen; D006801:Humans; D008024:Ligands; D046911:Macromolecular Substances; D008970:Molecular Weight; D010455:Peptides / Q000138:chemical synthesis / Q000378:metabolism; D010980:Platelet Membrane Glycoproteins / Q000302:isolation & purification / Q000378:metabolism*
D008024:Ligands; D046911:Macromolecular Substances; D010455:Peptides; D010980:Platelet Membrane Glycoproteins; D005340:Fibrinogen; D002118:Calcium